If you're looking to access Affinity V2 from another app on Windows - for example, using Affinity Photo V2 as an External Editor in Raw Therapee, you will need to point to the app's execution alias. They can be found here (assuming default installation location):
Affinity Photo V2:
C:\Users\[USERNAME]\AppData\Local\Microsoft\WindowsApps\AffinityPhoto2.exe
Affinity Designer V2:
C:\Users\[USERNAME]\AppData\Local\Microsoft\WindowsApps\AffinityDesigner2.exe
Affinity Publisher V2:
C:\Users\[USERNAME]\AppData\Local\Microsoft\WindowsApps\AffinityPublisher2.exe